Group III-V semiconductor materials continue to be selected for use in the fabrication of various light sensitive, light emitting and electronic devices. The ultimate operation and performance of these devices is dependent upon the amount of leakage current present. Leakage currents are those which bypass the desired current path such as the active region in a device. In devices such as buried heterostructure (BH) semiconductor lasers, for example, leakage currents lead to high lasing threshold, low differential quantum efficiency, abnormal temperature dependence on threshold current, and rollover of the light-current (L-I) characteristic. All of these factors stemming from leakage currents have a serious negative impact on the use of the lasers as transmitters in optical communication systems.
An effective approach for blocking the flow of leakage currents through undesired paths is to introduce a layer of high resistivity material into the semiconductor structure. Previously, high resistivity liquid phase epitaxial (LPE)Al.sub.0.65 Ga.sub.0.35 As (lightly Ge-doped) material has been utilized for current confinement in AlGaAs/GaAs buried heterostructure (BH) lasers, but subsequent analogous attempts to produce high resistivity LPE InP material for this purpose in InGaAsP have not been successful. Deuteron bombardment has also been shown to produce highly resistive material from p-type InP, but this material is not expected to remain highly resistive during subsequent processing. In particular, because the high resistivity is related to deuteron implant damage, the resistivity anneals out at the high temperatures (e.g., above about 600.degree. C.) required for subsequent LPE growth.
In addition, reverse-biased p-n junctions have also been reported for constraining current to flow through the active region of InGaAsP/InP lasers. These blocking junctions have been fabricated by the implantation of Be into n-InP substrates, by the diffusion of Cd into n-InP substrates, and by the epitaxial growth of a p-InP layer onto an n-InP substrate. But, all of these devices are impaired to some extent by leakage currents because of the imperfect blocking characteristics of the reverse-biased junctions.
More recently, D. P. Wilt et al. reported in Applied Physics Letters, Vol. 44, No. 3, p. 290 (Feb. 1984) that InP/InGaAsP CSBH lasers with relatively low leakage currents and low lasing thresholds can be fabricated by incorporating into the structure a high resistivity Fe-ion-implanted layer which constrains pumping current to flow through the active region. The high resistivity layer is produced by an Fe-ion implant into an n-type InP substrate followed by an annealing treatment prior to LPE growth. This laser is also the subject of copending application Ser. No. 549,160 filed on Nov. 8, 1983 by R. J. Nelson et al. Although the resistivity of the Fe-ion-implanted layer is stable even after being subjected to the high temperatures characteristic of LPE growth, the thinness of the Fe-implanted layer (about 0.4 .mu.m) renders it difficult to reproducibly position the thin active layer (about 0.1-0.2 .mu.m thick) adjacent thereto. When the active layer is not so placed, shunt paths are created which allow leakage current to flow around the active layer. Hence, high performance (low threshold, high efficiency) devices are hard to fabricate reproducibly.
More recently, it has been found that reproducible BH lasers with low leakage currents, low lasing thresholds, excellent high frequency response and good reliability can be fabricated by incorporating into the structure a relatively thick, high resistivity Fe-doped InP-based layer grown by metallo-organic chemical vapor deposition (MOCVD) using either a ferrocene-based or iron pentacarbonyl-based dopant precursor. Importantly, InP:Fe layers which are relatively thick (e.g., 1-4 .mu.m) and highly resistive (e.g., 10.sup.3 -10.sup.9 .OMEGA.-cm) are realized by this process, characteristics which are crucial to reducing leakage currents and increasing yields in a variety of devices.
While iron doping of indium phosphide is useful for producing high resistivity, semi-insulating semiconductor material, the resulting material has poor thermal stability. Moreover, since iron is a deep acceptor in indium phosphide and because the semi-insulating material is grown in contact with a p-n junction, the semi-insulating material is susceptible to being rendered conductive in the vicinity of the p-type material because rapidly diffusing p-type impurities such as zinc, cadmium, magnesium, and beryllium change the net carrier concentration from an excess of shallow donors toward an excess of shallow acceptors. This has, in turn, caused the search to continue for other dopants to form semi-insulating indium phosphide. Although a large number of alternate transition metal dopants (Co, Cr, and Mn) have been studied for use with indium phosphide, none has achieved a successful combination of good semi-insulating behavior and thermal stability.
Recently, it was reported that titanium doping of bulk indium phosphide resulted in high resistivity semiconductor material which also exhibited good thermal stability. The semi-insulating bulk crystals were grown by liquid encapsulated Czochralski techniques using pyrolytic boron nitride crucibles. See C. D. Brandt et al., Appl. Phys. Lett., Vol. 48, No. 17, pp.1162-4 (1986). The high purity titanium source used for liquid encapsulated Czochralski growth is not suited for vapor phase or molecular beam epitaxial growth techniques. Moreover, the results fail to suggest a titanium source suitable for such epitaxial growth techniques which would be capable of producing semi-insulating indium phosphide exhibiting deep donor levels which result from titanium doping as opposed to deep acceptor levels associated with iron doping. While the reported results indicate that titanium doping is more desirable than iron doping in forming semi-insulating indium phosphide, the titanium source and growth techniques applied are incapable of producing or overgrowing semi-insulating indium phosphide epitaxial layers necessary for device fabrication.